A preliminary study of the weathering mechanism of fossilized Cretaceous Hamipterus bones

Abstract

The discovery of Hamipterus tianshanensis had important scientific significance because three-dimensional preserved fossils can provide unique information about the reproduction, development, and evolution of pterosaurs. However, the pterosaur fossils exhibited violent weathering, including noticeable cracking and spalling, since the preservation environment changed dramatically after excavation, which severely influenced the preservation of these fossils and the scientific research conducted on them. To determine the weathering mechanism of these fossil bones, the samples were analyzed using X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy and energy dispersive spectrometry (SEM-EDS). The results indicate that calcite is the main mineral in the hollow bones, but it is mixed with a small amount of other minerals, such as quartz, feldspar, and other debris particles. Moreover, the main component of the pterosaur bones is hydroxyapatite, and carbonate has substituted for some of the phosphate (B-type carbonated apatite) in the fossil bones. A small amount of calcite and manganese oxides has precipitated in the vascular canals. The phosphate crystallinity index (PCI) is 4.17. These results indicate that the fossils have undergone severe diagenesis. Apart from these alterations, the fossil bones have high contents of anions and cations such as Cl−, NO3−, SO42−, Na+, and Ca2+, and the total content of soluble salts is 35584.41 μg g−1. Based on these results, it is inferred that the weathering mechanism of the fossil bones is as follows. First, the fragile hollow bones cracked easily because of the enormous thermal stress caused by the dramatic temperature changes in the Gobi Desert and the differences in the thermal expansion coefficients (TEC) of the surrounding rocks, the fossil bones, and the different fillers. Second, a large amount of stress is generated by salt crystallization when the temperature and humidity change, and the pterosaur fossils will be damaged when the crystallization pressure exceeds the tensile strength of the fossils. These results suggest that the preservation of pterosaur fossils requires suitable desalination and consolidation conditions. Moreover, they should be preserved in a constant temperature and humidity environment.